A2.2.1 Describe the functions and practical applications of network topologies.
• Network topologies: star, mesh, hybrid
• Factors to consider must include reliability, transmission speed, scalability, data collisions, cost.
• Examples may include home and small office settings, where reliability is paramount, and the use of networks in larger settings (e.g. corporations, government departments, college campuses).
The big idea
A network topology is the pattern in which nodes (computers, access points, switches, routers) are interconnected.
That pattern is not just a wiring diagram—it determines how fast frames move, how faults propagate, how many devices you can add, and how much it all costs.
Three topologies dominate practical LAN design today: star, mesh, and hybrid (a deliberate mixture of the first two).
1 Star topology
| Aspect | How it works | Impact |
|---|---|---|
| Structure | Every edge device connects to a single central switch or wireless access point. | |
| Reliability | Failure of one spoke cable ≠ outage for others; failure of the central switch is catastrophic. | |
| Transmission speed | Each link is its own collision-free full-duplex segment (e.g., 1 Gb s⁻¹ twisted-pair Ethernet). | |
| Scalability | Limited by port count on the hub/switch; adding a second aggregation switch scales easily. | |
| Data collisions | None on modern switched Ethernet; MAC scheduling is per-port. | |
| Cost | One extra cable per device; inexpensive 8- to 48-port switches keep capital cost low. |
Practical applications
- Homes & SOHO – Wi-Fi access point in star with four-port switch; reliability hinges on the single box, which is acceptable for non-critical sites.
- Classroom or lab – one 48-port switch per rack; easy to diagnose link faults.
2 Mesh topology
| Aspect | How it works | Impact |
|---|---|---|
| Structure | Nodes have multiple redundant paths to one another; links may be wired, fibre, microwave, or wireless mesh (802.11s). | |
| Reliability | Very high: path diversity lets traffic detour around a failed link or node. | |
| Transmission speed | Aggregate throughput increases because traffic can flow across several parallel routes; per-link speed depends on medium. | |
| Scalability | Full mesh scales as n (n − 1)/2 links—impractical for large n; partial mesh (e.g., spine-leaf) scales linearly. | |
| Data collisions | None on switched fibre/ethernet; wireless mesh shares spectrum, so airtime contention may rise. | |
| Cost | Cabling, optics, and extra switch ports multiply rapidly; justifiable where uptime is critical. |
Practical applications
- Campus core – dual fibre uplinks from every distribution switch to two redundant core routers (partial mesh).
- Municipal wireless – self-healing Wi-Fi mesh nodes on streetlights maintain service if one gateway loses power.
3 Hybrid topology
A hybrid combines the low-cost simplicity of stars on the edge with the robustness of a mesh in the backbone.
| Layer | Typical arrangement | Rationale |
|---|---|---|
| Access | Star (PCs → edge switch; IoT sensors → Wi-Fi AP) | Minimises cabling and switch count. |
| Distribution / Aggregation | Partial mesh (two-tier spine-leaf, ring, or dual-star) | Provides alternate paths; isolates failures. |
| Core / Data-centre fabric | Clos, folded-Clos, or full mesh | Non-blocking east–west bandwidth for servers. |
Practical applications
- Government campus – edge buildings wired in star to local closets; closets interlinked in a ring to two head-end routers for redundancy.
- Enterprise data centre – rack top-of-rack (star) switches uplink in a leaf-spine mesh giving predictable 3:1 or 1:1 oversubscription.
4 Choosing a topology: factor-by-factor
| Factor | Star | Mesh | Hybrid |
|---|---|---|---|
| Reliability | Single point of failure at hub | Path diversity; highest fault-tolerance | Fault-tolerant core, acceptable edge risk |
| Transmission speed | Sized per edge port (1 G / 2.5 G / 10 G) | Parallel links boost aggregate BW | Core can be 40 G / 100 G while edges stay modest |
| Scalability | Add ports or stack another switch | Grows quickly in cables/ports | Scales by adding leaves to spine |
| Data collisions | None on switched links | None on wired; possible radio contention in wireless mesh | Same as constituent layers |
| Cost | Lowest | Highest | Moderate—spend where it matters |
Key take-aways
- Star is the default for small, cost-sensitive environments where single-device reliability is “good enough.”
- Mesh pays for itself in environments where downtime is unacceptable or where high aggregate throughput justifies multiple paths.
- Hybrid topologies dominate modern enterprise and campus networks, striking a balance between cost at the edge and redundancy in the backbone.
By matching the topology to the required reliability, speed, and budget, network architects ensure that a home Wi-Fi network, a college campus, and a multinational enterprise each get the resilience and performance their users expect—without overspending on unnecessary links.